In recent years, the time for printing process and development processing operations for print production have been shortened and speeded up, and there has been an increased demand for high speed photographic materials stability during processing, and handling durability.
The most common method for increasing the speed of a silver halide emulsion involves increasing the grain size, thereby increasing the amount of light which can be absorbed per grain. In those cases where the emulsion is color sensitive, an increase in speed can also be achieved by increasing the extent of light absorption of the sensitizing dye in such a way that photo-electrons are transmitted to the silver halide and linked to latent image formation. However, satisfactory results have not always been achieved using these methods. That is, increasing the grain size has an inhibiting effect on increasing the speed of the development process, and color sensitization not only inhibits development and de-silvering but normally reduces the remaining margin for any increase in speed with an increased amount of sensitizing dye. Hence, any method in which the speed of the silver halide grains is without increasing grain size or increasing the amount of sensitizing dye would be very useful. The method known as chemical sensitization is typical of such methods. Known such methods include those in which sulfur sensitizing agents such as sodium thiosulfate are used; those in which gold sensitizing agents such as potassium chloroauric acid are used; those in which reduction sensitizing agents such as stannous chloride are used; and methods in which combinations of these methods are used. Although the photographic speed which can be obtained using the above chemical sensitization methods is dominated by the type and quantity of sensitizing agent used, by the method of addition, and by the combination which is used, they are not the only determining factors and it is known that different results are observed depending on the nature of the silver halide grains themselves prior to chemical sensitization. For example, the way in which sulfur sensitization proceeds differs according to the habit of the silver halide crystal grains is discussed on pages 181-184 of the Journal of Photographic Science, Vol. 14 (1966) and, moreover, the efect of crystal habit on latent image formation when reduction sensitization is also carried out is discussed on pages 249-256 of volume 23 (1975) of the same journal. Furthermore, the relationships between the type of halide and the crystal habit of the halide, used for forming the emulsion grains, and the effect on photographic speed and fogging of sulfur sensitization and gold-sulfur sensitization carried out using the emulsified grains, is discussed on pages 146-149 of Photographic Science and Engineering, volume 28 (1984). However, these reports are concerned only with the effect of the nature of the silver halide grains on chemical sensitization and photographic speed. They provide no information regarding techniques and procedures for responding to the commercial demand for increased speeds and handling stability.
Methods of achieving higher speeds without increasing the silver halide grain size have been proposed for silver halide photographic materials. Furthermore, a further increase in handling strength and processing stability can be anticipated by increasing the photographic speed.
The formation of silver halide grains using so-called "halogen conversion" is proposed in JP-B-50-36978 and is one method for increasing the photographic speed of a silver halide. (The term "JP-B as used herein signifies an "examined Japanese patent publication".) The silver halide emulsions obtained using this method are seen to have an increased photographic speed and they have a further advantage in that the extent of fogging due to mechanical pressure is reduced. However, the inventors have discovered that these emulsions also have serious defects. That is, even though, the level of fogging is produced by mechanic pressure is reduced, there is a pronounced desensitization when parts which have been subjected to a mechanical pressure are exposed to light. The extent of halogen conversion can be reduced to minimize the extent of pressure desensitization, but this increases fogging due to pressure. Thus there are problems with fogging and desensitization due to pressure, and the two are incompatible. Furthermore, silver halide converted emulsions of this type have also been found to have softer gradation.